Conventionally, in performing various operations such as welding, painting, cleaning, energy beam radiation and machining using an industrial robot, a position of operation line is detected by a sensor supported on a robot hand and a path taught through on-line or off-line is corrected based on a signal from the sensor, to control the robot so that the actual tool path conforms with the ideal path. The work lines include a characteristic line capable of being identified by a sensor such as a weld line formed on a workpiece (for example, a boundary line formed by the difference in height between two workpieces to be welded) formed on a workpiece and a contour line of workpiece. The aforementioned sensor may be of a type such that the operation line position is detected by deflecting laser beam.
As a typical example of the application of such control method, the real-time tracking of weld line is known. The conventional procedure of real-time tracking control includes the following steps: first, the sensed data representing the weld line is acquired by the sensor, the acquired sensed data is converted to coordinate data on the coordinate system set on the robot, and the data converted into coordinate data is stored in memory means. When the tool tip point almost has approached the sensing position of a sensor, the coordinate data is read, and it is compared with the position data (interpolation point position data) of corresponding taught path segment to calculate an amount of correction .DELTA..
In the state in which the tool tip point has reached a position close to the subsequent taught point, the sensing region of the sensor has already advanced beyond the taught point, and has entered a region in which the weld line corresponding to the next taught path segment is present. At this point, to execute the above calculation processing by reading the data stored in the memory means, the time available for reading position data of the next taught point and for the subsequent interpolating calculation processing (calculation processing to determine the data of the next taught path segment) is not ample enough.
That is, when the tool tip point approaches the weld line corresponding to the next taught path segment, in order to determine the amount of position correction .DELTA., it is necessary to calculate many interpolation point position data of the next taught path segment and to perform, in advance, processing for providing correspondence to the data read from the memory means. Therefore, the total processing load including the calculation of position correction amount .DELTA. following the above processing and the calculation processing of target position based on the position correction .DELTA. becomes very high, so that it is difficult to issue the target position command timely before the tool tip point reaches the next taught point in order to control the robot so that the tool tip point is surely ready for tracking.
The above-described problem of the prior art is not limited to the welding work in which the weld line is tracked, but is commonly encountered when work is done in a fashion that a robot is moved while correcting the target position with respect to the taught path by using the result of sensing within the region in the tool travel direction.